1. Field of the Invention
The present invention generally relates to a user scheduling and beamformer design method, apparatus, and storage medium for multi-user multiple-input and multiple-output (MIMO) downlink based on two-stage MIMO beamforming as well as conventional single-stage MIMO beamforming.
2. Description of the Related Art
In wireless communication, the demand for high data rates has increased, and the necessity for the improvement of frequency efficiency has rapidly increased. In order to meet such necessities, a massive MIMO system in which a large number of antennas are employed into a base station to improve frequency efficiency is regarded as important technology for next-generation communication systems, and research into massive MIMO systems has actively been conducted all over the world.
In particular, in the situation of multi-user MIMO downlink in which a single base station simultaneously transmits data to multiple users, a method of efficiently scheduling users who will be actually provided with services, among all users in the cell who desire to be provided with the services, has been regarded as one of standardization-related important issues in Long-Term Evolution (LTE) and LTE-A (advanced) systems, and in-depth research into such methods is on-going. Conventional technologies related to the research are described below.
When the channels of all users are isotropically generated without having correlations, known user scheduling methods include user scheduling methods based on random beamforming disclosed in “On the capacity of MIMO broadcast channels with partial side information” by M. Sharif and B. Hassibi, IEEE Transactions on Information Theory, vol. 51, no. 2, pp. 506-522, 2005, and zero-forcing beamforming based on full channel state information (CSI) at the base station disclosed in “On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming” by T. Yoo and A. Goldsmith, IEEE Journal on Selected Areas in Communications, vol. 24, no. 3, pp. 528-541, 2006. The random beamforming-based scheduling method has the advantage of significantly reduced feedback from users to the base station and yields reasonable performance. Hence, the random beamforming method has been extended and introduced even to environments in which channels are spatially correlated.
Recently, massive MIMO adopting large antenna arrays in base stations are gaining interest as a promising technology for future wireless communication systems for very high data rates, and user scheduling for massive multi-user MIMO is a hot research topic. As disclosed in “Joint spatial division and multiplexing: Opportunistic beamforming and user grouping” by A. Adhikary and G. Caire, arXiv: 1305.7252, May 2013, a very simple user scheduling method for massive multi-user MIMO downlink is recently proposed by extending and modifying an existing random beamforming method proposed by Sharif and Hassibi to an environment in which two-stage beamforming is used for massive MIMO. In the two-stage beamforming, users are partitioned into several subsectors (or groups) by the 1st stage-beamformer and users within a subsector are separated by the 2nd stage beamformer. Since in two-stage beamforming the 1st stage beamformer is designed based on channel statistics information and the 2nd stage beamformer is designed based on the effective channel composed of the product of the actual MIMO channel and the 1st beamformer, the channel state information (CSI) for the second stage beamformer design is much reduced.
The random beamforming method applied to the conventional single-stage beamforming or two-stage beamforming case proposes a user scheduling and beamformer design method that is optimal only from the criterion of sum data rate scaling. Hence, it can be considered optimal only when a very large number of users are present, but it is not optimal in terms of sum rate itself and its sum rate performance degradation is severe in the range of the realistic range of user number. Hence, a new innovative scheduling and beamformer design method for multi-user MIMO downlink to overcome the drawback of the random beamforming is necessary.